The U.S. National Research Council's report entitled Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond identified the need for a middle-term space mission of ASCENDS (NRC, “Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond,” The National Academies Press, Washington, D.C., 2007.) The primary objective of the ASCENDS mission is to make CO2 column mixing ratio measurements during the day and night over all latitudes and all seasons and in the presence of thin or scattered clouds. These measurements would be used to significantly reduce the uncertainties in global estimates of CO2 sources and sinks, to provide an increased understanding of the connection between climate and atmospheric CO2, to improve climate models, and to close the carbon budget for improved forecasting and policy decisions. (NRC, “Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond,” The National Academies Press, Washington, D.C., 2007.)
The ASCENDS mission requires an active sensor that has the needed CO2 column measurement precision from space to determine the global variability of CO2 in the troposphere and thereby determine CO2 sources and sinks at the surface. The critical component in the ASCENDS mission is the Laser Absorption Spectrometer (LAS) for CO2 column measurements with weighting towards the mid to lower troposphere using Integrated Path Differential Absorption (IPDA) technique.
IPDA lidar systems measure the difference of the total gas absorption along the path length between two or more laser wavelengths in the path of the laser to a target. For the earth's climate system, there are large changes in the atmospheric variables such as pressure and temperature profiles, clouds, aerosols, and the profiles of water vapor and other absorbing gases from local spot to global scales. Also, the surface reflectance varies with many factors and variables such as surface type, roughness, slope, vegetation, and soil moisture. The variations in these atmospheric and surface factors and variables would generate large changes in received lidar powers from the surface or other targets, which would introduce huge problems in determination of CO2 column amounts when individual lidar wavelengths are investigated. The advantage of IPDA LAS system is that the strong effects of the variations in atmospheric states and surface reflection on lidar return powers would be eliminated from the ratio of received lidar powers of two close wavelengths. All the influences on the lidar power from these environmental variables are the same for the two or more close wavelengths. The ratio removes all environmental effects except the differential absorption optical depth at the studied LAS wavelengths. Thus, high accuracy of CO2 column measurements (within 1 ppm) can be achieved from the CO2 differential optical depth.
Basic CO2 IPDA LAS measurement approaches include pulsed (Grady J. Koch, Bruce W Barnes, Mulugeta Petros, Jeffrey Y Beyon, Farzin Amzajerdian, Jirong Yu, Richard E Davis, Syed Ismail, Stephanie Vay, Michael J Kavaya, Upendra N Singh. “Coherent Differential Absorption Lidar Measurements of CO2,” Applied Optics, Vol. 43 Issue 26, pp. 5092-5099 (2004) doi: 10.1364/AO.43.005092) and (James B. Abshire, Haris Riris, Graham R. Allan, Clark J. Weaver, Jianping Mao, Xiaoli Sun, William E. Hasselbrack, S. Randolph Kawa “Pulsed airborne lidar measurements of atmospheric CO2 column absorption,” Presented at the 8th international carbon dioxide conference, ICDC8, in Jena Germany 13-19 Sep. 2009, Tellus B, 62: 770-783, doi: 10.1111/j.1600 0889.2010.00502.x) and CW systems. Direct detection pulsed systems use a technique where measurements for each wavelength are done sequentially through separate columns. This works well but errors can occur if there are differences in reflectivity and solar background radiation from shot to shot and one wavelength to the next. Also, the atmospheric air mass measured for pulse systems is different from one wavelength pulse to other wavelength pulses. The CW approach, however, transmits lidar powers of different wavelengths in the same aligned laser beam, fundamentally senses the same column of atmosphere and spot of surface, and thus, removes the potential errors for pulse systems. Within the CW approach, IM techniques are used to separate the signals from different wavelengths, provide ranging capability, and discriminate surface returns from multiple other returns such as those of thin clouds, for instance. They are also useful for separating the laser signals from background solar returns, which occurs in a natural way through the matched filter processing techniques used. Potential IM-CW techniques include pn code CW (Joel F. Campbell, Narasimha S. Prasad, Michael A. Flood “Pseudorandom noise code-based technique for thin-cloud discrimination with CO2 and O2 absorption measurements,” Opt. Eng. 50(12), 126002 (Nov. 18, 2011), doi:10.1117/1.3658758) and (Joel F. Campbell, Michael A. Flood, Narasimha S. Prasad, and Wade D. Hodson “A low cost remote sensing system using PC and stereo equipment,” American Journal of Physics, Vol. 79, Issue 12, pp. 1240, December 2011), linear swept sine wave CW (R. Agishev, B. Gross, F. Moshary, A. Gilerson, and S. Ahmed “Atmospheric CW-FM-LD-RR ladar for trace-constituent detection: a concept development,” Appl. Phys. B 81, 695-703(2005), doi: 10.1007/s00340-005-1919-x), (Oscar Batet, Federico Dios, Adolfo Comeron, and Ravil Agishev “Intensity-modulated linear-frequency-modulated continuous-wave lidar for distributed media:fundamentals of technique,” Applied Optics, Vol. 49, No. 17, pp. 3369-3379, 10 Jun. 2010 doi: 10.1364/AO.49.003369), (Masaharu Imaki, Shumpei Kameyama, Yoshihito Hirano, Shinichi Ueno, Daisuke Sakaizawa, Shuji Kawakami, Masakatsu Nakajima “Laser absorption spectrometer using frequency chirped intensity modulation at 1.57 μm wavelength for CO2 measurement,” Optics Letters, Vol. 37, No. 13, pp. 2688-2690, 1 Jul. 2012 doi: 10.1364/OL.37.002688), (Edward V. Browell, J. T. Dobler, S. A. Kooi, M. A. Fenn, Y. Choi, S. A. Vay, F. W. Harrison, B. Moore III “Airborne laser CO2 column measurements: Evaluation of precision and accuracy under wide range of conditions,” Presented at Fall AGU Meeting, San Francisco, Calif., 5-9 Dec. 2011) (Edward V. Browell, J. T. Dobler, S. A. Kooi, M. A. Fenn, Y. Choi, S. A. Vay, F. W. Harrison, B. Moore III “Airborne validation of laser CO2 and 02 column measurements,” Proceedings, 16th Symposium on Meteorological Observation and Instrumentation, 92nd AMS Annual Meeting, New Orleans, La., 22-26 Jan. 2012. https://ams.confex.com/ams/92Annual/webprogram/Paper197980.html), and (Jeremy T. Dobler, F. Wallace Harrison, Edward V. Browell, Bing Lin, Doug McGregor, Susan Kooi, Yonghoon Choi, and Syed Ismail “Atmospheric CO2 column measurements with an airborne intensity-modulated continuous wave 1.57 μm fiber laser lidar,” Applied Optics, Vol. 52, Issue 12, pp. 2874-2892 (2013)), unswept sine wave CW (Songsheng Chen, Yingxin Bai, Larry B. Petway, Byron L. Meadows, Joel F. Campbell, Fenton W. Harrison, Edward V. Browell “Digital Lock-in detection for multiple-frequency intensity-modulated continuous wave lidar,” 26th International Laser Radar Conference, S1P-38, Porto Heli, Greece, Porto Heli, Greece, 25-29 Jun. 2012), (Shumpei Kameyama, Masaharu Imaki, Yoshihito Hirano, Shinichi Ueno, Shuji Kawakami, Daisuke Sakaizawa, Toshiyoshi Kimura, Masakatsu Nakajima “Feasibility study on 1.6 mm continuous-wave modulation laser absorption spectrometer system for measurement of global CO2 concentration from a satellite,” Applied Optics, Vol. 50, No. 14, pp. 2055-2068 (2011), doi: 10.1364/AO.50.002055) (Michael Dobbs, Jeff Pruitt, Nathan Blume, David Gregory, William Sharp “Matched Filter Enhanced Fiber-based Lidar for Earth, Weather and Exploration,” NASA ESTO conference, June 2006 http://esto.nasa.gov/conferences/estc2006/papers/b4p3.pdf). (Dobbs, M. E., J. Dobler, M. Braun, D. McGregor, J. Overbeck, B. Moore III, E. V. Browell, and T. Zaccheo “A Modulated CW Fiber Laser-Lidar Suite for the ASCENDS Mission,” Proc. 24th International Laser Radar Conference, Boulder, Colo., 24-29 Jul. 2008). and (Jeremy T. Dobler, ITT Exelis, Fort Wayne, Ind.; and J. Nagel, V. L. Temyanko, T. S. Zaccheo, E. V. Browell, F. W. Harrison, and S. A. Kooi “Advancements in a multifunctional fiber laser lidar for measuring atmospheric CO2 and O2,” Proceedings, 16th Symposium on Meteorological Observation and Instrumentation, 92nd AMS Annual Meeting New Orleans, La., 22-26 Jan. 2012. https://ams.confex.com/ams/92Annual/webprogram/Paper202790.html), non-linear swept sine wave CW (Joel F. Campbell “Nonlinear swept frequency technique for CO2 measurements using a CW laser system,” Applied Optics, Vol. 52, Issue 13, pp. 3100-3107 (2013)), and other similar techniques. The baseline technique used by NASA Langley Research Center in partnership with ITT Exelis for the ASCENDS mission is linear swept frequency CW (Jeremy T. Dobler, F. Wallace Harrison, Edward V. Browell, Bing Lin, Doug McGregor, Susan Kooi, Yonghoon Choi, and Syed Ismail “Atmospheric CO2 column measurements with an airborne intensity-modulated continuous wave 1.57 μm fiber laser lidar,” Applied Optics, Vol. 52, Issue 12, pp. 2874-2892 (2013)). Experiments have demonstrated the utility of this technique for both range and CO2 column absorption measurements. An advantage of this technique is that by simultaneously transmitting orthogonal online and offline signals together through a single column of atmosphere, one may make a simultaneous online/offline lidar return power measurement, thereby minimizing error caused by variations in surface reflectivity and reflected solar background radiation.
One potential issue with some modulation techniques, such as linear swept frequency, is the potential for side lobes, which could introduce small bias errors when intermediate scatterers are close to measurement targets. This is an advantage of digital techniques such as pseudorandom-noise (PN) codes and non-linear frequency modulation. A known technique uses a pure maximum length (ML) sequence with time shifting to separate channels (Joel F. Campbell, Narasimha S. Prasad, Michael A. Flood “Pseudorandom noise code-based technique for thin-cloud discrimination with CO2 and O2 absorption measurements,” Opt. Eng. 50(12), 126002 (Nov. 18, 2011), doi:10.1117/1.3658758). This technique may have the advantage of eliminating the side lobe issue for certain hardware configurations in LAS systems, while allowing for multiple channels that share the same bandwidth. Additionally, the time shifting approach makes the data processing of multi-channel laser returns very simple since one only needs to perform a single matched filter correlation for all channels instead a separate matched filter correlation for each channel. One potential disadvantage with this technique is that not all hardware has a suitable low frequency response, which applies to the transmitter as well as the receiver. Since a ML sequence has a main power band at zero frequency, using that technique in such hardware will filter out the main power band and ruin the autocorrelation properties, which can result in non-orthogonality in a time shifted multi-channel configuration. This problem could be solved by putting the PN code on a carrier, performing amplitude demodulation, and conducting matched filter correlation with the reference PN code to obtain the range capability. A low cost demonstration of this was implemented using Hilbert Transforms forms for amplitude demodulation. That system had better autocorrelation properties but still exhibited minor artifacts and the demodulated PN code exhibited minor distortions and ringing (Joel F. Campbell, Michael A. Flood, Narasimha S. Prasad, and Wade D. Hodson “A low cost remote sensing system using PC and stereo equipment,” American Journal of Physics, Vol. 79, Issue 12, pp. 1240, December 2011).